Industrial printing systems are used to print images onto large volumes of substrates such as paper. The images are often stored as a digital image that is sent to the printing system to be printed. It is often important to an operator of such printing systems that any defects in the printing process be detected so that the problem can be quickly fixed before subsequent defective images are printed. A defect is any unwanted discoloration, marking, or characteristic of the printed image.
One way to check for such defects is to scan the printed image and compare the digital scanned image with the original digital image. However, the tolerances for variation between the scanned image and the original digital image have to be relatively high in order to account for expected discrepancies. These discrepancies arise due to a variety of factors including the inability of physical ink to perfectly match the digital colors and the inability of the scanner to perfectly capture the colors of the scanned image. Other discrepancies include spatial misalignment that occurs as the paper moves through its path during the scanning process. Due to the higher tolerances, various defects such as low contrast defects often go undetected. These low contrast defects, while difficult to detect with standard comparison functions, may be quite visible to the human eye.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.